PALEOZOIC GROUP. 567 



in McCulloch County. This division is but imperfectly represented along the 

 southern border of the Central Mineral Region. Its rocks are crystalline and 

 shaly limestones or dolomites, somewhat fossiliferous in parts, with two prom- 

 inent beds made up largely of sponges and other similar remains. It is con- 

 venient to make two subdivisions, but we have not been able to give them 

 the attentive study which they must receive before they can be safely correlated 

 with the strata of other States. Excepting as building stones and road metal 

 (and possibly as a source of quicklime in some cases) the economic value of 

 the Hinton beds is not very great. 



a. The Birdseye Subdivision. — The base of the Hinton is sometimes marked 

 by one transitional bed, or more, of siliceous limestone, with a layer of mot- 

 tled, limestone simulating a calcareous conglomerate. Overlying this is a 

 pink, white, or variegated limestone with crystalline dots or facets. This is 

 very similar to what is known as the "Birdseye" limestone in New York 

 State, and its stratigraphic position is certainly closely related to that stratum 

 in the Eastern United States. On Cold Creek, Llano County, this layer is 

 well exposed in the canyon above Bauman's, especially near the top of the 

 ridge lying west of the creek. Overlying the birdseye rock, in complete sec- 

 tions, there are peculiar crystalline gritty dolomites, sometimes fossiliferous. 

 For practical uses these may be temporarily regarded as belonging to the 

 same subdivision, although very close study may eventually make a different 

 arrangement desirable. 



b. The Sponge Subdivision. — The upper portion of the Hinton division 

 may readily be distinguished from all other terranes by the immense masses 

 of fossil sponges of which the principal beds are composed. The lower 

 stratum contains globose forms, often of very large size, which Mr. Nagle has 

 aptly termed "hat sponges," from their peculiar appearance when the rock 

 has become water worn, as in the bed of Cold Creek, Llano County, at Bald- 

 win's ranch; also in the lower course of Bluff Creek, Mason County, in sev- 

 eral places ; and in the bed of Deer Creek, above the mouth of Hinton Creek, 

 in San Saba County. 



2. The Deep Creek Division. — This may be termed the chert division 

 of the Silurian. There are other cherty horizons in the Carboniferous and 

 Cretaceous, but one who becomes familiar with the thick and peculiar beds 

 of the Silurian will not often mistake the other coutcrops for them. There 

 is a black chert in the Carboniferous which is sometimes regarded as coal by 

 ignorant people, but the Silurian chert is usually massive, of light blue or 

 grayish tint, and ordinarily not nodular, as is much of that in the Cretaceous 

 recks. Some of the beds are very thick. The set comprises a variety of 

 rocks, all highly siliceous, some weathering roughly, others honeycombed and 

 more or less crystalline. The upper layers are spongiform, with drusy crystals 



